Petrus Old Brown beer is a blend of the brewery’s Aged Pale, aged for up to two years in huge oak casks, with a fresh dark beer. During this aging process, just like in wine, lactic acids develop. These acids are responsible for the refreshing sourness in the taste. Only tasting on a regular basis can tell the brewer when the beer is ready to come out of the oak barrels and to be blended with fresh brewed beer. This blend is then bottled.

Petrus Old Dark ripens for 24 months in oak barrels, this gives its vinous, noble taste and slightly sour in character.